Component casting is used in order to produce a wide range of components and members. Essentially, the component is cast in a mould from a molten liquid and then allowed to cool in order to leave a solidified component. Some components such as turbine blades for jet engines require structural abilities such as high temperature creep resistance. This is achieved with turbine blades through forming a single crystal. At high temperatures, typically above half the absolute melting temperatures of the metal, the grain boundaries become weaker than the grain bodies such that the absence of such grain boundaries in a single crystal provides resistance to creep.
Techniques for producing single crystal components are well known. Essentially the component is cast in a mould and then gradually withdrawn from a furnace in an appropriate manner such that propagation of a single crystal is achieved. Typically, a so called “pig-tail” selector is used in order to initiate a single grain or crystal growth. The most important consideration with respect to continued propagation of a single crystal within the component is to ensure so called directional solidification. This is achieved by gradual withdrawal, usually downwardly of the component from the furnace such that the temperature gradient is effectively controlled.
Generally, the interface temperature between the solid and liquid must be slightly lower than the melting point of the solid and the liquid temperature must increase beyond the interface. To achieve this temperature gradient, the latent heat of solidification must be conducted through the solidifying solid crystal. In any event, ideally the temperature interface should be flat and gradually progress through the component in order to ensure a uniform single crystal is provided with few, if any, defects at the interface. Particular problems can be experienced for instance at relatively thin and overhanging parts of the component, where the material may tend to solidify too quickly, and tend to grow its own grains.
It should also be understood that the solidus/liquidus mix or mushy zone between the solid component and the liquid material should be rendered as stagnant as possible. Unfortunately, most components by their nature are shaped and so provide differing radiation heat effects due to the varying thickness of the component at particular points. These changes render it difficult to fully control the temperature gradient and therefore an unacceptable proportion of components are rejected due to defects formed during casting.